The method for preparing isocyanate by primary amine and phosgene in gas phase is well known. In the method, primary amine enters into a gas phase phosgenation reactor after vaporization, and reacts with phosgene to produce isocyanate. The effect of vaporization of primary amine has a significant influence to the reaction product.
In the real industrial production operation, the problem of incomplete vaporization appears frequently during the vaporization of primary amine, and amine droplets that are not vaporized present in the amine gas stream formed after vaporization. These amine droplets will enter into the gas phase phosgenation reactor with the amine gas stream and cause unfavorable consequences if no corresponding measures are taken to remove them. In one aspect, as the diameters of the amine droplets are relatively large and the droplets are heated unevenly, the amine molecules on the surface of the droplets will react with the phosgene to produce isocyanates, while under high temperature the amine molecules inside the droplets will be carbonized to produce carbon deposit and ammonia gas, and the ammonia gas will react with hydrogen chloride (one of the products of the reaction of amine and phosgene) to produce solid ammonium chloride, and the carbon deposit and ammonium chloride will easily cause the blocking of the reactor and pipes, thus frequently cleaning up will be needed, the running periods of the device will be shortened. In another aspect, undesired side reactions often happen between the amine molecules inside these droplets and the isocyanates produced on the surface of the droplets, resulting in the increase of heavy component impurities in the reaction product and the decrease of yield.
At present, there are two methods for removing the amine droplets contained in the amine gas stream. The first method is to remove droplets by gas-liquid separation, generally, with a relative large pressure loss in this method. The second method is to let amine droplets vaporize by heating to obtain amine gas stream that does not contain amine droplets. However, the present heating method has the disadvantages of large pressure loss and unevenly heating. During the removing of amine droplets in the amine gas stream, if the pressure loss is too large, the pressure during amine vaporization will be increased, and there will be a rise of the temperature for vaporization, which will cause amine to decompose thus produce ammonia gas. The ammonia gas produced enters into the phosgenation reactor then reacts with the hydrogen chloride (one of the products of the reaction of amine and phosgene) to produce solid ammonium chloride, which will easily cause the blocking of the reactor inner wall and pipes, and the running periods of the device will be shortened. If the heating method is used, there may be the problem of unevenly heating, which will easily cause the circumstance that the heating temperature is too high or too low locally, and even cause the circumstance that the heating temperature of some parts is too high while the temperature of other parts is too low. When the heating temperature is too high locally, the amine molecules in the amine droplets will be carbonized during the removing of amine droplets, which leads to the formation of carbon deposit and ammonia gas thus blocking the device; when the heating temperature is too low locally, some of the amine molecules react with the obtained isocyanates to produce heavy component impurities as they cannot reach the temperature required for the phosgenation reaction after the amine gas stream enters into the phosgenation reactor.
In EP1935876A1, amine gas streams that contain basically no droplets are generated before entering into the reactor, which makes the continuous running period of the reactor increase significantly. The patent document mentioned that the removal of amine droplets in amine gas stream can be performed in a droplet separator provided between the amine vaporization system and the overheating system, and/or by using a vaporization device with the function of droplet removal itself, and it is mentioned that the droplet remover with less pressure loss is preferred. However, said method is disadvantageous for large scale industrialization of the gas phase phosgenation reaction, as the pressure loss of the droplet separator will dramatically increase with the increase of volume flow rate. Under the same pressure loss, the separation rate of the amine droplets of the droplet separator under high volume flow rate is lower than that under low volume flow rate, therefore the effect of removing amine droplet is worse. If the separation rate of the amine droplets of the droplet separator is increased by means of increasing the pressure loss, the pressure during amine vaporization will be increased, thus causing the above mentioned blocking of the device, and shortening the running period of the device.
CN102471242A provides a method for removing droplets that are not vaporized after the vaporization of amine. Said method allows a small amount of droplets-containing amine that obtained after the removal of most of the droplets of the vaporized amine gas stream by the droplet separator or not by the droplet separator to be overheated in the inlet pipe of the guide reactor, and it is required for the residence time of the overheating process to be longer than 0.01 second to promote the vaporization of the contained droplets, thus finally forming a completely vaporized flow. The overheating of the vaporized amine gas stream in the inlet pipe can be performed in a device such as a tube bundle heat exchanger or a heating pipe. But during the actual operation process, if droplet separator is used, there will be large pressure loss, and the blocking of the device and the shortening of the running period of the device mentioned above will be caused. If the vaporized amine gas stream is overheated directly inside the inlet pipe, instead of using droplet separator, as the temperature of the material close to the pipe wall of the inlet pipe is much higher than that of the center of the pipe, the vaporized amine gas stream will be unevenly heated, and the amine molecules inside the amine droplets in the amine gas stream close to the pipe wall under high temperature will be carbonized to produce carbon deposit and ammonia gas. While the temperature of the amine gas stream in the center of the pipe is low, thus it will produce heavy component impurities after entering into the phosgenation reactor. In addition, the inlet pipe for overheating must be longer than ever in order for the amine droplets in the amine gas stream to be sufficiently vaporized. The residence time of the amine gas stream under high temperature is prolonged, which will easily cause the decomposition of the amine to produce ammonia gas, and consequently more ammonium chloride will be formed in the reactor. Thus the blocking of the downstream devices will be accelerated, and the running period of the devices will be shortened.
CN101912751A discloses an amine vaporizer that is made of inert inorganic conductive material, wherein the inert inorganic conductive material possesses irregular microporous channels. Liquid amine enters into the amine vaporizer to vaporize after it is atomized and dispersed into droplets. The vaporizer is used for the vaporization of the amine droplets obtained via atomization, not for the removing of the amine droplets contained in the amine gas stream obtained via vaporization. Even if the vaporizer is used as the heater for the removing of the amine droplets contained in the amine gas stream, the pressure loss generated during the vaporization of the amine droplets will be large, which will easily cause the blocking of the devices and the shortening of the running period of the devices mentioned above, since a microporous channel structure is used as the main body of the vaporizer, which has long channels and low porosity.
In addition, the droplet size of the droplets in the amine gas stream generated during the amine vaporization is generally between 0.1 mm and 1 mm. In conditions where the effect of vaporization is poor, droplets with a droplet size between 1 mm and 2 mm will appear. The larger the droplet size, the longer the time needed to vaporize and heat the droplets, and heating under high temperature for long time will cause the amine to decompose thus result in the above undesired result.
Therefore, during the process of preparing isocyanate by gas phase phosgenation, a device for removing the amine droplets in the amine gas stream is needed, with low pressure loss or basically no pressure loss, and evenly heating.